(1) Field of the Invention
The present invention relates to mobile terminal equipment and a packet communication method between terminals and, more particularly, to mobile terminal equipment to which a mobile IP (Internet Protocol) is applied and a packet communication method between terminals in a mobile communication system.
(2) Description of the Related Art
In recent years, application of an IP (Internet Protocol) to a mobile communication network is being actively studied. For example, as described in “Mobility Support in IPv6 <draft-ietf-mobileip-ipv6-18.txt>, Work in Progress”, the IETF (Internet Engineering Task Force) is standardizing the Mobile IPv6 specification. Mobile IPv6 basically inherits the functions of a mobile IP conformed with IPv4 specified in IETF RFC3220.
A mobile IPv6 network is comprised of a mobile node MN, a home agent HA, and a correspondent node CN. To the mobile node MN, an permanent IP address (home address) which does not change even when the mobile node MN moves is assigned. A link having the same prefix value as the home address is called a home link. When moved to a link other than the home link, the mobile node MN obtains an IP address to be used in the foreign link. The address is called a care of address (CoA).
The mobile node MN receives a router advertisement signal periodically sent from a router installed in a foreign link. When it is detected that the received router advertisement signal has a prefix value different from the home address or the present CoA, the mobile node MN recognizes that it moved to another link. When the mobile node MN detects that it has moved to a new link, the mobile node MN transmits a location registration request (BINDING UPDATE) message to the home agent HA.
On reception of the control message (BINDING UPDATE), the home agent HA stores the binding information of the home address and the care of address CoA of the mobile node MN indicated in the received message into a binding cache management table. After that, to intercept packets addressed to the mobile node MN, the home agent HA multicasts a control message (GRATUITOUS NEIGHBOR ADVERTISEMENT) to neighboring routers and, thereafter, operates as a proxy server of the mobile node MN.
To communicate a message in the mobile IPv6 specification between the mobile node MN and the correspondent node CN, the node CN also has to be adapted to mobile IPv6. In the mobile IPv6 specification, for example, packet transmission from the node CN to the mobile node MN is carried out in the following procedure.
The node CN transmits a packet to the home address of the mobile node MN. The packet is intercepted by the home agent HA. After intercepting the packet addressed to the home address, the home agent HA retrieves the care of address CoA corresponding to the home address from the binding cache management table, adds an IP header (encapsulation) having the CoA as a destination address to the packet, and transmits the encapsulated received packet to a network.
The packet addressed to the CoA is transferred to the link where the mobile node MN exists and is received by the mobile node MN. Upon receiving the packet, the mobile node MN removes the IP header (decapsulation) added by the home agent HA, thereby obtaining the original packet transmitted from the correspondent node CN. At this time, the mobile node MN searches a binding update list on the basis of the IP address of the transmission source node CN indicated in the received packet. The binding update list is a table provided in the mobile node MN for storing transmission destination information of a binding update message.
When there is no entry for the node CN in the binding update list, the mobile node MN transmits a control message (BINDING UPDATE) for notifying the node CN of a correspondence relation between the home address of the mobile node MN and the care of address CoA.
Upon receiving the binding update message, the node CN registers the correspondence relation between the home address of the mobile node MN and the CoA indicated in the received message into a binding cache table of the node CN. It enables the node CN to transmit a packet, which is generated after that and destined to the mobile node MN, through an optimum communication path by applying the care of address CoA indicated in the binding cache table as the destination address of the packet. An IP packet transmitted from the node CN to the mobile node MN includes the home address of the mobile node MN in an extension header for controlling the IPv6 route.
As described in, for example, “Route Optimization in Mobile IP <draft-ietf-mobileip-optim-11.txt> Work in Progress”, IETF is working on route optimization in mobile IPv4. The route optimization in mobile IPv4 is realized by sending a notification of CoA corresponding to the home address of the mobile node MN from the home agent HA to the correspondent node CN.
On the other hand, in the field of the IP network, VOIP (Voice over IP) technique of transmitting voice by IP packets is also being examined. In the VOIP, a virtual communication path or session is established between communication apparatuses prior to start of communication, and voice data is transferred in the form of IP packets on the session. Establishment, maintenance, and disconnection of the session between the communication apparatuses is controlled according to a session control protocol.
IETF has specified an SIP (Session Initiation Protocol) for establishing and releasing a session in an IP multimedia communication (IETF RFC3261). Attention is being paid to the SIP as a session control protocol of VoIP because of high extensibility of the function.
The SIP is an application protocol using transport mechanism such as the TCP (Transmission Control Protocol) or UDP (User Datagram Protocol). The SIP is a text-base protocol and an SIP message is comprised of a header carrying a request or response and a message body describing the session contents. To the session description for SIP, for example, SDP (Session Description Protocol) is applied (IETF RFC2327 and IETF RFC3266).
Since the SIP employs the architecture of a client/server model. An origination client transmits an SIP request to a proxy server (SIP server) of a destination client. The SIP server specifies the address of a destination node by using, for example, DNS (Domain Name System) or the like and establishes a session between the client nodes.
The operation modes of the SIP server are classified into a proxy mode and a redirect mode according to the role of the SIP server. In the proxy mode, a request of establishment of a session between the origination client and the destination client is relayed by the proxy server. In the redirect mode, the origination client obtains information of the destination client from the SIP server and directly communicates with the destination client.
In the case where a terminal X and a terminal Y in an IP network performs voice communication via the SIP server in the proxy mode, the terminal X transmits a call establishment request (INVITE) message to the SIP server in advance of communication with the terminal Y. Upon receiving the call setting request message (INVITE), the SIP server specifies location information of the terminal Y, and transfers the received message (INVITE) to the terminal Y. The terminal Y having received the call establishment request transmits a response message (200 OK) indicative of the acceptance of the call. The response message is transmitted to the terminal X via the SIP server through which the call establishment request was relayed.
Upon receiving the response message, the terminal X transmits a response acknowledge (ACK) message to the terminal Y. The response acknowledge (ACK) message is transmitted via the SIP server or directly from the terminal X to the terminal Y. When the terminal Y receives the response acknowledge (ACK), a session is established between the terminals X and Y. Usually, each of the call establishment request message and the response message includes session description information necessary for transferring data packets (voice packets) between the terminals X and Y. The terminal X (terminal Y) transmits data packets to a destination address designated in the session description by the correspondent terminal Y (terminal X).
In the SIP, the other party is identified on the basis of an SIP URI (SIP Uniform Resource Identifier). Each client registers its location information, for example, IP address in a registrar. The registrar transmits the location information received from the client to a location server, and the location server stores a correspondence relation between the SIP URI and the location information of the client into an SIP information management table. The SIP server may be provided with the functions of the registrar and the location server.
When the mobile node MN of the mobile IP moves in a communication network (hereinbelow, called visited network) away from the home network of the mobile IP, the mobile node MN notifies the mobile IP home agent HA of the care of address CoA obtained in the visited network, whereby the home agent HA operates as a proxy of the mobile node MN. The correspondent terminal CN transmits a packet to the home address of the mobile node MN. The packet is intercepted by the home agent HA and encapsulated with the IP header including the care of address of the mobile node MN as the destination address. The encapsulated packet is transferred to the mobile node MN.
That is, a data packet transmitted from the correspondent node CN (or mobile node MN) to the mobile node MN (or correspondent node CN) is transferred through the home agent HA until the process of optimizing the communication route of the mobile IP is completed between the mobile node MN and the correspondent node CN. In this case, since the communication between the mobile node MN and the home agent HA is performed in a form of an encapsulated packet obtained by adding an encapsulation IP header to an original packet, overhead for packet transfer control increases. It causes a problem such that delay time until a packet arrives at a destination terminal increases.
When the communication route optimizing process is executed during communication of data packets, a difference occurs between packet transfer time before route optimization and packet transfer time after route optimization. These problems are more serious in the case of providing VOIP service in the mobile IP communication network.